Oil country tubular goods are used for extracting and producing crude oil or natural gas. An oil country tubular good has its both end threaded and further oil country tubular goods are added as the oil well or gas well is drilled to a deeper level. At the time, the oil country tubular good is subjected to stress by its own weight. Therefore, the oil country tubular good must have high strength. Deeper oil wells or gas wells have been drilled, and 110 ksi grade oil country tubular goods (having a yield strength from 758 MPa to 861 MPa) have been used recently and development of 125 ksi grade oil country tubular goods (having a yield strength of 861 MPa to 965 MPa) is under way.
Such oil country tubular goods for use in oil wells or gas wells must have high resistance against sulfide stress cracking (hereinafter referred to as “SSC”). The SSC is generated because of stress acting on steel used in a hydrogen sulfide environment and the SSC resistance generally decreases as the strength of the steel increases. Therefore, improvement of the SSC resistance is crucial for oil country tubular goods having high strength.
Reported methods for improving the SSC resistance of a high strength oil oil country tubular good include the following approaches.
(1) To highly clean the steel.
(2) To quench the steel and then temper the steel at high temperatures.
(3) To refine the crystal grains of the steel. The steel is for example quenched twice or subjected to induction heating, so that the crystal grains are refined.
(4) To control the morphology of carbide generated in the steel. More specifically, to refine or/and spheroidize the carbide.
In the disclosure of JP 2000-313919 A or International Publication 00/68450 pamphlet, steel is made to have a homogeneous martensite structure by reducing its Cr content and carrying out direct quenching, so that the SSC resistance of the steel for high-strength oil country tubular goods can be improved.
As described above, improvement of steel has been focused on improvement of the internal quality of the steel. However, high strength oil country tubular goods provided with the above-described countermeasures sometimes still suffer from SSC.